Method for recovery of heavy metals from highly concentrated wastewater solutions

ABSTRACT

Heavy metals are reclaimed from a concentrated wastewater solution containing heavy metal cations and an acid by adding to the wastewater a bed of scrap aluminum in a total amount between about 160% and about 180% of the stoichiometric requirement for complete reduction of the dissolved heavy metal cations to their elemental states and adjusting the acid content of the wastewater to 5% to 20% by volume and allowing the heavy metal cations to react with the scrap aluminum to oxidize the aluminum and to reduce the heavy metal cations to their elemental states thereby producing demetallized wastewater which is neutralized with caustic to produce a treated effluent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Heavy metals occur as cations in acidic (and, occasionally, alkaline)wastewaters produced during manufacture of printed circuit boards and inother industrial processes. Some wastewater streams containing coppercations are both highly concentrated (that is, in excess of severalthousand ppm by weight of metal) and very acidic (pH below 1.0). Copper,lead and tin in particular may be recovered in saleable form from thesewastewater streams by contacting them with scrap aluminum, anotherbyproduct of the manufacture of PC boards, in an aqueous solution ofsulfuric acid. Other metals of Groups IB, IIB, IIIA, and IVA of thePeriodic Table also can be recovered using the process and apparatus ofthin invention, to the extent cations of such metals may be present inwastewaters being treated. In the process of this invention, controlled,stepwise addition of aluminum during the reduction process, coupled withcontrol of the acid concentration, improves reaction rates, reduces thedanger of thermal runaway and limits oxidation of aluminum compared toconventional processes.

The invention relates to recovery of Group IB, IIB, IIIA and IVA metalsfrom acidic or alkaline wastewaters produced in various manufacturingindustries. More particularly, it relates to treatment of wastewaterstreams produced during PC board manufacturing operations to recovercopper, lead and tin therefrom in saleable from, while reducing theconcentration of toxic metal cations in the treated wastewater to levelssafe for discharge to municipal sewer systems.

2. Description of the Prior Art

Plating of PC boards and subsequent steps in PC board manufactureproduce some wastewater streams that contain very high concentrations ofCu⁺² cations. These streams come from pickling acids and electrolesscopper solutions, and may contain from 1,000 ppm to as high as 500,000Cu⁺² by weight. They typically also contain high concentrations of H₂SO₄, typically enough to produce pH levels of 1.0 or less. In somesituations, such CU⁺² -containing streams may be alkaline Additionally,washing steps that follow the etching process produce wastewater streamscontaining lower concentrations of Cu⁺² cations, typically in the 20 ppmto 50 ppm range. Later washing steps produce wastewater streamscontaining Pb and Sn cations that result from subsequent solderingoperations. Federal, state and local environmental regulations prohibitdischarge of wastewaters containing Cu, Pb and Sn cations because oftheir toxicity; discharge levels of 1 ppm or below are considereddesirable, and in some states such as California discharge levels as lowas 0.4 ppm may be required. Discharge of highly acidic wastewaters tomunicipal sewers is also undesirable. Accordingly, various methods ofreducing the cation content of these wastewaters and producing anear-neutral treated stream have been proposed.

A customary method of treating these wastewaters is by addition of largeamounts of alkali salts, such as NaOH·Na₂ CO₃ or KOH, which causesprecipitation of a floc of Cu(OH)₂. One such process is described inU.S. Pat. No. 3,816,306 (Roy). While this treatment also neutralizes thewastewater, it leaves Na+ or K+ cations in solution and produces largevolumes of unsalable floc, essentially trading one disposal problem foranother.

Another method of purification uses ion exchange to replace Cu⁺² cationswith less-toxic cations in solution. But ion exchange techniques alsoproduce unsalable spent ion exchange media that must be disposed of inlandfills or otherwise.

A more promising approach is to contact the wastewater containing metalcations including those of Cu, Pb or Sn with a more electropositive,non-toxic metal in elemental form. In this manner, the toxic cations arereduced to elemental state, a form in which they have value asbyproducts, and the wastewater is cleansed of toxic metal cations.Because elemental aluminum is another byproduct of PC board manufacture,and aluminum is both low in toxicity and more electropositive than Cu,Pb or Sn, it is commonly suggested for recovery processes of this sort.

One such process is described in detail in U.S. Pat. No. 4,304,599(Durkee). In that continuous process, wastewater containing Cu⁺² cationsis adjusted to a pH of 2 or less by addition of sulfuric acid, and thencontacted in stagewise fashion with aluminum tailings retained betweenscreens in a series of reaction cells through which the wastewater ispumped. Overall wastewater residence time is about three hours. A verylarge excess of aluminum is employed; in the example given by Durkeeroughly 50 times the stoichiometric requirement for reaction with theCu⁺² cations being processed. (Even the first cell of the Durkeeapparatus alone contains almost three times the stoichiometricrequirement of elemental aluminum.) Following reaction with thealuminum, the wastewater contains about 50 ppm of Cu⁺². It is then mixedwith other waste streams that are substantially copper free, reducingthe Cu⁺² concentration to about 1.25 ppm. Finally, it is neutralizedwith caustic and discharged. Details of the reaction cell layout usedfor this process are described in U.S. Pat. No. 4,294,434 (Durkee).

In another version of the Durkee process, described in U.S. Pat. No.4,450,002 (Durkee), certain modifications to the process allow operationat pH levels in the range of 3.2 to 3.3, thus reducing sulfuric acidrequirements. This is achieved by adding a centrifuge to the dischargeend of the series of reaction cells. The centrifuge removes very finelydivided particles of copper which would otherwise escape in theeffluent. The '002 patent also teaches that much slower reaction ratesare achieved at the higher pH ranges used in this process, so thatadequate conversion of Cu⁺² may require recycle of part of the effluentstream back to the inlet of the series of reaction cells. As in theprocess described in the '599 patent, very large excesses of aluminumare used in comparison to stoichiometric requirements.

Yet another continuous process involving recycle of wastewatercontaining Cu⁺² is described in U.S. Pat. No. 3,905,827 (Goffredo etal). In that process, continuous radial flow through a large fixed bedof aluminum turnings is used to purify rinsewater so that it can bereused over and over again in the PC board manufacturing process. As inboth versions of the Durkee process, a large excess of aluminum isnecessarily used. And like the Durkee process, the Goffredo process isnot suitable for treating wastewater streams containing highconcentrations of metal cations.

The Durkee processes recover saleable copper, but they have significantdrawbacks. Use of aluminum in many times stoichiometric quantities iswasteful (scrap aluminum has substantial resale value itself). Undesiredside reactions between the aluminum and the sulfuric acid increase theconcentration of soluble Al⁺³ cations in the treated effluent. While notparticularly toxic, such cations nevertheless are not desirable intreated water. In treating wastewater streams with high concentrationsof Cu⁺² cations, such as pickling acids and electroless coppersolutions, use of large excesses of aluminum poses a danger of thermalrunaway because of the exothermicity of the Cu - Al redox reaction. Infact, the Durkee process is designed for continuous processing oflow-concentration wastewater streams, and will not operatesatisfactorily for batch processing of wastewater streams containinghigh metal cation concentrations. (Although U.S. Pat. '002 to Durkeeteaches application of the process to highly-concentrated wastewaterstreams (col. 10, lines 7-11), no specific examples are provided, andexperimentation has shown that the Durkee process, with its large excessof aluminum, fails to operate satisfactory on high-concentration wastestreams.) Moreover, the Durkee process actually reduces Cu⁺²concentrations in the wastewater treated in the reaction cells only tolevels around 50 ppm. In order to allow discharge to the sewer system atlevels in the 1 ppm range, the treated wastewater is simply diluted withother wastewater that does not contain any copper. Federal and stateenvironmental authorities generally will not accept wastewater treatmentprocesses that can meet discharge concentration standards only bydiluting an otherwise non-complying waste stream with clean water inorder to reduce the concentration of pollutants to within acceptablelimits. Thus, the Durkee process does not achieve substantially completerecovery of copper from the wastewater stream actually treated. TheDurkee process does not provide for separate recovery of Pb and Sn fromrinsewater streams that are also produced during other stages of PCboard manufacture. Nor does Durkee provide for recovery of oxidizedaluminum from the treated wastewater.

There is a need for a process and apparatus that safely achievessubstantially complete copper recovery without need for final dilution,even when the feedstock contains very high levels of Cu⁺² and/or othermetal cations; that minimizes both the consumption of aluminum and thelevel of Al⁺³ cations in the treated effluent, and that can separatelyrecover Pb, Sn and other metals of Groups IB, IIB, IIIA and IVA insaleable form from wastewater streams containing those contaminants.

SUMMARY OF THE INVENTION

The process of this invention uses two to four reaction vessels to treathigh-concentration, metal-containing wastewater streams to recover themetals in saleable, elemental form, and to reduce the residual metalcation concentration to environmentallyacceptable levels withoutreliance on dilution with clean water. The process is particularlyapplicable to wastewater streams produced in the course of PC boardmanufacture, including wastewaters from pickling operations, electrolesscopper solutions, and wastewater streams containing lead and tin cationsresulting from soldering operations. In the preferred embodiment of theinvention, as applied to wastewater streams containing Cu, Pb and Sn,the wastewater streams are accumulated in separate reactors having meansfor retaining a bed of scrap aluminum above their bottoms, and alsohaving liquid withdrawal pumps capable of continuously recirculating thewastewater from the bottom of the reactor to the freeboard above theliquid level in the top of the reactor. When the reactors are full,recirculation of the liquid is begun and a controlled amount of aluminumscrap is added-- typically, 80% to 90% of the theoretical stoichiometricrequirement for complete reduction of the copper or lead/tin rations totheir elemental states. The temperature of the liquid is monitored andheld below 180 F. and above 100 F. by addition of ice or sulfuric acidas needed. (In the descriptions that follow, reference is made to use ofsulfuric acid by way of example, but those skilled in the art willunderstand that a range of mineral acids of appropriate strength can besubstituted therefor without departing from the scope of thisinvention.) Sulfuric acid content is adjusted to within the range of 5%to 10% by volume. About 30 minutes after the reaction begins, additionalaluminum scrap is added to bring the total aluminum present into therange of 160% to 180% of stoichiometric requirements for completereduction of copper or lead/tin cations to their elemental states.Copper and lead/tin contents are monitored periodically, with additionalcontrolled quantities of aluminum added if the reaction ceases beforeCu⁺² and Pb/Sn cation contents reach acceptable levels of about 0.5 to1.0 ppm or below.

When acceptable heavy metal cation levels have been achieved in theliquid, recirculation is stopped and the contents of both tanks arepumped together into a third reactor, to which sufficient caustic(usually NaOH) is added to raise the pH of the treated wastewater toroughly 7. A relatively small amount of Al(OH)₃ precipitates out of theliquid at this point and is retained on the perforated false bottom ofthe pH adjustment reactor. The liquid is then pumped out of the reactorinto a filter press, and then to a final pH adjustment tank (the fourthreactor) where its pH is balanced to neutral (7.0) by addition of smallamounts of caustic or sulfuric acid as required. From there, the treatedwastewater is pumped through a "slim line" type filter, and dischargedto the municipal sewer.

Al(OH)₃ from the pH adjustment reactor is pumped out in a water slurryand recovered in a filter press. The supernant water is pumped to thefinal pH adjustment tank to mix with the offtake from the pH adjustmentreactor.

After the copper and Pb/Sn conversion reactors have been drained, theremaining untreated scrap aluminum and elemental copper or lead and tinis repeatedly washed. Initial washing is with water; subsequently a 2%NaOH solution is used, and finally water again. Water from thesewashings goes to the pH adjustment reactor. Finally, untreated scrapaluminum and saleable Grade 2 copper and lead/tin amalgam are removedfrom the conversion reactors and the treatment process is ready forrepetition.

Accordingly, it is an object of this invention to provide an improvedmethod for recovering metals of Groups IB, IIB, IIIA and IVA fromaqueous waste streams. More particularly, it is an object of thisinvention to provide an improved method for recovering copper, lead andtin in their elemental states from pickling acid, electroless coppersolutions, and rinsewaters from etching and soldering operations used inPC board manufacture.

It is a further object of this invention to provide a process andapparatus that will operate safely on wastewater streams having veryhigh concentrations of metal cations, and in particular of Cu⁺² cations,without danger of thermal runaway.

It is a further object of this invention to recover elemental metalssuch as copper, lead and tin in their elemental states, in saleable formwhile conserving and eventually recovering the scrap aluminum used as areducing agent.

Another object of this invention is to minimize the quantity of Al⁺² andAl⁺³ cations released to municipal sewage systems in the treatedwastewater.

Still another object of this invention is to provide a process that willreduce copper, tin and lead cation concentrations in the treatedwastewater to the range of 0.5 to 1.0 ppm or below, suitable fordischarge to municipal sewage systems, without need for a last stage ofdilution with cation-free water, a procedure that is environmentallyunacceptable.

Another object of this invention is to provide a four-reactor systemusing simple batch processing tanks that is capable of treating bothcopper-containing and lead/tin containing wastewaters, in which aluminumscrap consumption is minimized and heavy metal recovery is facilitated.

Another object of this invention is to provide simplified twoandthree-reactor systems for sequential recovery of different metals fromwastewater streams, in which one metal recovery reactor is usedalternately for more than one type of metal.

Further objects and advantages of this invention will be apparent tothose skilled in the art from a review of the following drawings anddescription of the preferred four-reactor embodiment and otherembodiments of the invention, as well as the claims. It will be apparentto those skilled in the art that the invention is not limited to theembodiments described below, but that other combinations of apparatus,and variations on processing parameters, also can be used to practicethis invention without departing from the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram indicating the method of the preferredembodiment of the invention in schematic form.

FIG. 2 is a flowsheet illustrating the principal pieces of equipmentused to practice the invention in its preferred fourreactor embodiment.FIG. 3 is a detail showing internal construction features of the twoconversion reactors and the first pH adjustment reactor, shown in FIG.2.

FIGS. 4 and 5 are flowsheets illustrating the principal pieces ofequipment used to practice the invention in a simplified form using onlythree reactors

FIG. 6 is a flowsheet showing the most important items of equipment usedto practice the invention in a further simplified way using only tworeactors.

DESCRIPTION OF THE FOUR-REACTOR PREFERRED EMBODIMENT

Referring initially to FIGS. 1 and 2, the first step in the process isto collect copper-containing wastewater in copper recovery reactor 1,and lead and tin-containing wastewater in lead/tin recovery reactor 2.In the four-reactor preferred embodiment, the recovery reactors are 500gallon capacity fiberglass tanks with wall thicknesses ranging from 3/8inch at the top to 1/2 to 3/4 inch at the bottom. The copper-containingwastewater preferably consists of spent solution from acid picklingoperations and electroless copper solution, so that the mixed wastewaterin the tank already contains sufficient sulfuric acid that its pH isbelow 1.0 and the Cu⁺² cation content is around 10,000 ppm. (To thoseskilled in the art, however, it will be apparent that alkalinewastewaters also can be treated by acidifying them before going on tosubsequent processing steps.) If the Cu⁺² cation content divergessignificantly from the 10,000 ppm range, the amounts of scrap aluminumstated below can be adjusted proportionately, so as to maintain thestated relationships between aluminum used and theoretical,stoichiometric requirements for reduction of the Cu⁺² cations to theirelemental state.

When the reactors are full, pumps 3 and 4 are started, recirculating thewastewater from the bottom of the reactors back through the overheadpipes 5 and 6 into the tops of the reactors. The reaction is started byloading ten pounds of aluminum scrap (about 80% to 90 % of thestoichiometric requirement for complete conversion of the copper cationsto their elemental states) into each of the conversion reactors throughtop hatches 7 and 8. At the same time, about 5 gallons of spent etchback sulfuric acid or any other spent sulfuric acid is also pumped intoeach of the two reactors. The scrap aluminum immediately sinks to theperforated false bottoms 9 and 10 of the reactors, which are preferablyslightly concave upward. The scrap aluminum is retained on the falsebottoms 9 and 10 above the perforated, fiberglass horizontal liquidwithdrawal pipes 11 and 12. Withdrawal pipes 11 and 12, which arepreferably about 18 inches in diameter, extend across the plenums formedbetween the bottom heads of the reactors 13 and 14 and the perforatedfalse bottoms 9 and 10. Inside the withdrawal pipes are concentriccylindrical filters 15 and 16, which consist of perforated fiberglasspipes about 3 inches in diameter covered by poly filter bags havingabout 5 micron pores. The filters 15 and 16, like the withdrawal pipes11 and 12, extend entirely across the bottoms of the reactors 1 and 2.

After the first charge of aluminum scrap has been added, the temperatureof the wastewater in the reactors is monitored and held between 100 F.and 180 F. by adding ice if necessary to cool the liquid, or moresulfuric acid if necessary to heat it. Approximately 30 minutes afterthe first charge of aluminum scrap has been added, a second charge ofabout 10 lbs. of aluminum scrap is added to each of the reactors,bringing the total amount of aluminum added into the approximate rangeof 160% to 180% of the theoretical stoichiometric amount needed forcomplete reduction of the metal cations in solution to their elementalstates. Reduced copper, lead and tin form on the aluminum scrap andlikewise are retained on the false bottoms 9 and 10 of the reactors. Thecirculating wastewater is sampled and analyzed for sulfuric acid contentusing techniques known in the art, such as addition of methyl orangefollowed by titration with a base.

No sooner than 30 minutes after addition of the second charge ofaluminum scrap, sufficient additional sulfuric acid is added to bringthe acid content of the circulating wastewater into the range of 5% to10% sulfuric acid by volume. It has been found that acid concentrationsas high as 20% by volume are feasible, but optimum results are obtainedin the range of 5% to 10% because below 5% the rate of reaction isalmost imperceptible, and above 20% there is considerable danger ofboiling of the wastewater and thermal runaway. At very high sulfuricacid concentrations, it may be necessary to reduce the amount of scrapaluminum used in order to avoid thermal runaway. Moreover, use ofsulfuric acid concentrations above 20% results in large increases in thecost of NaOH needed for neutralization. Recirculation of the wastewaterensures that the metal-containing wastewater flows downward through thebed of aluminum scrap supported on false bottoms 9 and 10, eventuallyexposing all of the wastewater to repeated contact with the aluminumscrap.

Beginning about 24 hours after the first charge of aluminum scrap hasbeen added, the recirculating wastewater is sampled periodically andanalyzed for metal cation content (typically Cu, Pb and Sn, although asnoted above other metals of Groups IB, IIB, IIIA and IVA may berecovered as well) using techniques known to those skilled in the art,such as atomic absorption spectroscopy. If the reaction stops before thedesired level of metal cation content has been attained (typically about0.5 to 1.0 ppm), an additional 10 pound charge of scrap aluminum may beadded. The target level of 0.5 to 1.0 ppm of metal cations usually isattained within one to four days, so that sampling is appropriatelyconducted on a once-a-day basis.

After the target level of metal cation content has been attained, thewastewater from reactors 1 and 2 is pumped together into pH adjustmentreactor 17 by pumps 3 and 4 through transfer pipes 18 and 19. The pHadjustment reactor 17 is constructed in the same fashion as metalrecovery reactors 1 and 2, including provision for recirculation ofliquid using pump 3, except that instead of a 3 inch filter inside of a18 inch perforated tube below its dished false bottom 20, the pHadjustment reactor simply has a 18 inch diameter perforated pipe 21. ThepH adjustment reactor 17 also may be built with a liquid capacityroughly twice that of the metal conversion reactors 1 and 2, so that itcan accommodate demetallized wastewater from both copper and lead/tinrecovery operations simultaneously.

When the demetallized wastewater has been pumped out of the metalconversion reactors 1 and 2, the remaining, unreacted scrap aluminum andrecovered copper and lead/tin mixture retained on the false bottoms 9and 10 of the reactors is washed with water and dilute caustic,preferably in the following sequence:

1. Three consecutive clean water washes.

2. Two washes with 1 to 3 wt% NaOH solution.

3. One final clean water wash.

Wash water from this operation is pumped into pH adjustment reactor 17.Finally, after the washing cycle is complete, manhole covers 22 and 23are opened and cleaned, saleable second grade copper, lead and tin aremanually recovered along with unreacted aluminum. Recovered metalsshould be air-dried before sale.

The wastewater in the pH adjustment reactor 17 is next neutralized byaddition of sufficient caustic to raise its pH to approximately 7. Thistypically causes precipitation of Al(OH)₃, which remains suspended inthe wastewater as a slurry or floc. After precipitation, the slightlybasic wastewater and precipitated Al(OH)₃ is pumped out of the pHadjustment reactor 17 through perforated pipe 21, which is unfiltered,by pump 3 through transfer pipe 24 and to filter press 25. The filterpress recovers precipitated Al(OH)₃, and filtered, demetallized waterpasses from the filter press into the final pH adjustment tank 26. FinalpH adjustment tank 26 also can receive wastewaters that do not containsignificant metal cation concentrations from other plant process, butthat require pH adjustment before discharge to the sewer system. In thetank, the liquid pH is measured by pH meter 27, which automaticallycontrols caustic supply pump 28 and acid supply pump 29 so as toneutralize the wastewater, producing a final neutral pH of 7.0. Finally,neutralized wastewater is pumped through a final stage of filteringusing a "slimline" type filter 30, and then discharged to the municipalsewer.

FIG. 3 shows construction details of the copper and lead/tin conversionreactors. Empty fiberglass tanks have been used in the past to recovermetallic copper from these sorts of wastewaters by reaction withelemental aluminum, and these tanks have included features such as theloading hatch 7, manhole 22, overflow outlet 32 and exhaust gas outlet33 (which is usually connected to the plant fume hood system to ventevolved hydrogen). But the internal structure shown in FIG. 3 is novel.Its primary features of interest are the dished, perforated false bottom9, which is mildly concave upward, and the underlying perforated 18 inchdiameter pipe 11 enclosing a 3 inch diameter perforated tube 15 which iscovered with a 5 micron filter bag. The false bottom 9 supports thealuminum scrap which reduces the metal cations, while allowing free flowof recirculating wastewater through the bed of aluminum scrap and intothe underlying plenum 31. The large perforated tube 15 extends all theway across the plenum 31 and screens out any small pieces of scrap thatmay fall through the false bottom 9. Inside the perforated pipe 11, a 3inch diameter tube covered with a 5 micron filter 15 acts as a liquidwithdrawal device, filtering out any smaller solid particles that mayhave gotten through the false bottom 9 and the larger pipe 11. (The endof the filter 15 that is furthest from the outlet flange 34 rests onpipe support means 35, which projects from the far wall of the reactor1.) In this way the filter 15 is not overloaded and choked by exposureto heavy concentrations of large metal particles, making sustained,continuous recirculation possible over the entire period of thereduction operation. (Vertical bag filters used in prior art conversionreactors tended to clog part way through the conversion process,reducing or stopping the recirculation of wastewater and therebysubstantially limiting mass transfer efficiencies.)

The following specific examples conducted under the conditions of theSummary of the Invention are illustrative of the operation of theinvention.

    ______________________________________                                        INITIAL METAL          FINAL METAL                                            CONCENTRATION          CONCENTRATION                                          PPM BY WEIGHT          PPM BY WEIGHT                                          ______________________________________                                        9600      ppm Cu.sup.+2                                                                              1.20    ppm Cu.sup.+2                                  5500      ppm Cu.sup.+2                                                                              0.91    ppm Cu.sup.+2                                  9000      ppm Cu.sup.+2                                                                              0.93    ppm Cu.sup.+2                                  15000     ppm Cu.sup.+2                                                                              0.21    ppm Cu.sup.+2                                  35000     ppm Cu.sup.+2                                                                              1.10    ppm Cu.sup.+2                                  9000      ppm Cu.sup.+2                                                                              0.90    ppm Cu.sup.+2                                  5600      ppm Cu.sup.+2                                                                              0.23    ppm Cu.sup.+2                                  300000    ppm Cu.sup.+2                                                                              0.54    ppm Cu.sup.+2                                  10000     ppm Cu.sup.+2                                                                              0.27    ppm Cu.sup.+2                                  1400      ppm Cu.sup.+2                                                                              0.26    ppm Cu.sup.+2                                                         642     ppm Al                                         13000     ppm Pb.sup.+2                                                                              0.50    ppm Pb.sup.+2                                  ______________________________________                                    

The tests reported above produced the results shown within reactiontimes of one to four days. In contrast, a test conducted on a high Cu⁺²concentration waste stream under conditions similar to those used in theDurkee process required eight days to reduce the Cu⁺² concentrationbelow 1 ppm. Moreover, the test used many times as much aluminum scrapas that required in the process of this invention.

DESCRIPTION OF A THREE-REACTOR EMBODIMENT

FIGS. 4 and 5 show how the process can be practiced on just onewastewater stream at a time using only three reactors instead of four.In FIG. 4, the metal conversion reactor 36 is constructed and operatedaccording to the same principles as the copper and lead/tin conversionreactors shown in FIGS. 2 and 3. It is first charged with wastewatercontaining one metal cation, such as copper, in significantconcentration, and operated according to the sequence shown in FIG. 1.When tests show that reduction of the metal cation is complete, thesolution is pumped out of the metal conversion reactor 36 by pump 37through a pleated filter 38 and into the pH adjustment reactor 39, wherecaustic is added while recirculating the solution through pump 40 toraise the pH to approximately 7, resulting in precipitation of a floc ofAl(OH)₃. Saleable metal and the remaining unreacted scrap aluminum isrecovered from metal conversion reactor 36 by washing and drying as inthe preferred embodiment described above. After precipitation, thesupernant liquid from the pH adjustment reactor 39 is pumped throughpump 40, to pleated filter 41 and thence into the final pH adjustmenttank 42. There the pH of the filtered effluent is adjusted to neutral(7.0), and subsequently discharged through slim line filter 43 to themunicipal sewer. Wastewater streams that do not contain significantconcentrations of metal cations also can be neutralized in the final pHadjustment tank 42 before discharge.

The three-reactor embodiment also can be configured as shown in FIG. 5.In this embodiment, neither the metal conversion reactor 44 nor the pHadjustment reactor 45 has a dished false bottom like that shown in FIG.3. Instead, scrap aluminum and reduced metal simply rest at the bottomof the reactor, and recirculation of liquid is accomplished by means ofdiaphragm pumps 46 and 47, which can tolerate some suspended solids inthe liquid being pumped. After metal reduction is complete, filter press48 is used to recover any suspended aluminum or reduced metal from theliquid as it is pumped to pH adjustment reactor 45. After cleaning, thesame filter press 48 is used to recover Al(OH)₃ from the approximatelyneutralized effluent from pH adjustment reactor 45. This embodiment usesreactors of simpler construction than those discussed above.

In both of the three-reactor embodiments, wastewater streams containingdifferent metal cations can be treated sequentially. For example, in theembodiment of FIG. 4, wastewater streams containing lead and tin cationscould be accumulated in a holding tank (not shown) while metalconversion reactor 36 was used to treat copper-containing wastewater;then the lead/tin wastewaters could be treated while copper-containingwastewater was stored. In this way the process of this invention may bepracticed using only two reactors with the internals shown in FIG. 3,instead of three such reactors as in the preferred four-reactorembodiment. The disadvantage is that larger reactors are requiredbecause of the need to accumulate wastewaters while alternately treatingdifferent wastewater streams.

DESCRIPTION OF A TWO-REACTOR EMBODIMENT

By combining the functions of the pH adjustment reactor with those ofthe final pH adjustment tank of the preferred four-reactor embodiment,and by treating only one type of metal-containing wastewater stream at atime, it is possible to practice this invention using only two reactorsFIG. 6 illustrates this embodiment of the invention. Here, a singlemetal conversion reactor 49 is constructed as shown in FIG. 3 andoperated as described in FIG. 1, as in the four-reactor preferredembodiment. After the reaction is complete, the treated wastewater ispumped out of metal conversion reactor 49 by pump 50 through pleatedfilter 51 or filter press 52 and directly into final pH adjustment tank53. That tank is equipped with mixer 54 to facilitate neutralization ofthe solution and to maintain the AL(OH)₃ floc in suspension. When the pHhas reached 7.0, the treated, neutralized wastewater is pumped from thepH adjustment tank 53 by pump 55 to a filter system 56, which issuitable for recovering Al(OH)₃ floc before discharge of the treatedwater to the sewer system. Like the three-reactor embodiments describedabove, this two-reactor embodiment can be combined with one or morewastewater storage tanks (not shown), and sized appropriately tosequentially process wastewater streams containing different dissolvedmetal cations.

The foregoing detailed descriptions of several embodiments of theinvention illustrate various methods and apparatuses suitable forpracticing the invention. Additional embodiments may be perceived bythose skilled in the art. In particular, with appropriate alterations inoperating conditions it should be possible to practice the invention ona continuous rather than a batchwise basis. Such additional embodimentsalso are within the scope of this invention.

We claim:
 1. A method of reclaiming heavy metal from a concentratedwastewater solution containing heavy metal cations and an acid,comprising the steps of:a. adding to said wastewater a bed of scrapaluminum in a total amount between about 160% and about 180% of thestoichiometric requirement for complete reduction of the dissolved heavymetal cations to their elemental states; b. adjusting the acid contentof said wastewater to 5% to 20% by volume; c. allowing said heavy metalcations to react with said scrap aluminum, whereby said aluminum isoxidized and said heavy metal cations are reduced to their elementalstates, and demetallized wastewater is produced; d. neutralizing saiddemetallized wastewater with caustic to produce a treated effluent. 2.The method of claim 1 wherein said acid is sulfuric acid.
 3. The methodof claim 1 wherein said heavy metal cations are selected from cations ofmetals of the group consisting of Groups IB, IIB, IIIA and IVA of theperiodic table of elements.
 4. The method of claim 2 wherein said heavymetal cations are selected from the group consisting of copper cationsand a mixture of lead and tin cations.
 5. The method of claim 4, whereinthe step of adding said scrap aluminum comprises the followingoperations:a. adding about one half of said total amount of scrapaluminum; b. adjusting the temperature of said wastewater to betweenabout 100 F. and 180 F., and c. adding the second half of said totalamount of scrap aluminum.
 6. The method of claim 5, wherein the step ofreducing said heavy metal cations further comprises recirculating saidwastewater through said bed of scrap aluminum until the concentration ofsaid heavy metal cations has been reduced to about 0.5 to 1 ppm.
 7. Themethod of claim 6, wherein the step of neutralizing said demetallizedwastewater comprises the following operations:a. decanting saiddemetallized wastewater from said scrap aluminum and adding sufficientcaustic to raise the pH of said demetallized wastewater to about 7; b.filtering out of said demetallized wastewater any aluminum hydroxidethat has precipitated out as a result of said increase in pH, and c.adjusting the pH of said filtered, demetallized wastewater to neutral(7.0), by adding caustic or sulfuric acid as required.
 8. The method ofclaim 7, further comprising the step of recovering elemental heavy metaland scrap aluminum.
 9. The method of claim 8, wherein said step ofrecovering elemental heavy metal and scrap aluminum comprises thefollowing operations:a. washing said scrap aluminum and elemental heavymetals three times with clean water; b. washing said scrap aluminum andelemental heavy metals twice with a 1% to 3% by weight aqueous solutionof NaOH; c. washing said scrap aluminum and elemental heavy metals onefinal time with clean water; and d. air drying said scrap aluminum andelemental heavy metals.
 10. A method of reclaiming heavy metal from aconcentrated wastewater solution containing sulfuric acid and heavymetal cations selected from the group consisting of copper cations and amixture of lead and tin cations, comprising the steps of:a. adding tothe wastewater scrap aluminum in an amount between 80% and 90% of thestoichiometric requirement for reducing the dissolved metal cations; b.adjusting the temperature of the wastewater to within the range of about100 F. to 180 F.; c. adding a second charge of scrap aluminum to saidwastewater in an amount sufficient to raise the total amount of aluminumto between 160% and 180% of the stoichiometric requirement for reducingthe dissolved metal cations; d. adjusting the sulfuric acid content ofsaid wastewater to about 5% to 10% by volume; e. recirculating saidwastewater through said scrap aluminum, allowing the reduction of heavymetal to continue until the heavy metal content of said wastewater hasdropped to 0.5 to 1.0 ppm, whereby elemental heavy metal anddemetallized wastewater is produced and some scrap aluminum remainsunreacted; f. decanting said demetallized wastewater from said elementalheavy metal and said unreacted scrap aluminum; g. recovering saidelemental heavy metal and unreacted scrap aluminum by rinsing saidelemental heavy metal and unreacted scrap aluminum by conducting thefollowing operations: (1) washing said unreacted scrap aluminum andelemental heavy metals three times with clean water; (2) washing saidunreacted scrap aluminum and elemental heavy metals twice with a 1% to3% by weight aqueous solution of NaOH; (3) washing said unreacted scrapaluminum and elemental heavy metals one final time with clean water; and(4) air drying said untreated scrap aluminum and elemental heavy metals;h. adjusting the pH of said demetallized wastewater to about 7 by addingcaustic, whereby aluminum hydroxide may be precipitated out of saiddemetallized wastewater; i. filtering said aluminum hydroxide, if any,out of said demetallized wastewater; j. adjusting the pH of saidfiltered, demetallized wastewater to neutral (7.0), and k. dischargingsaid filtered, demetallized wastewater to a municipal sewer.
 11. Themethod of claim 10, wherein two metal conversion reactors are usedsimultaneously to recover copper in one reactor and lead/tin in theother.
 12. The method of claim 10, wherein a single metal conversionreactor is used to sequentially treat a plurality of wastewater streams,each containing one or more different dissolved metal cations.